How to calculate it
List your brand and your main competitors. Pull monthly search volume for each brand name from any keyword tool. Your share is your volume divided by the total.
Track it monthly. The absolute number matters less than the direction.
Why it leads revenue
People search a brand name before they buy it, often weeks before. A rising share of search means more of the market is considering you, which shows up in sales later.
A falling share while your revenue holds is an early warning worth acting on.
Its limits
It works badly for very new brands with no measurable volume, for brands with generic names, and in categories where buying happens without search. Use it alongside direct traffic rather than alone.